108 THE PALEONTOLOGY OF MINNESOTA. 
(Distribution. 
GEOLOGICAL DISTRIBUTION. 
It is a singular fact that no remains whatever of Bryozoa are known from rocks 
of earlier date than the Chazy limestone of the Lower Silurian System. Here the 
class suddenly leaps into a prominence, not only in the way of individual represen- 
tation, but in the matter of diversity of structure, that is both surprising and difficult 
of explanation. Nor was it, as might be expected, the simpler types that prevailed 
here. On the contrary, it is the more complex types like the T’repostomata and Cryp- 
tostomata that are the most abundant and diverse in their development. What may 
be even more surprising is that every suborder known in the fossil state was repre- 
sented before the close of the Lower Silurian era. 7 
The vertical range of a few of the Lower Silurian genera (Stomatpora and Beren- 
icea), 18 likewise remarkable, and not equalled, so far as known, in any other class of 
animals, excepting the Brachiopoda, of which the genus Lingula, the same as the 
bryozoan genera alluded to, has living representatives. Still, as a rule, the vertical 
range of Bryozoa is restricted to comparatively narrow limits, and most genera and 
many families fail to pass from one system of rocks to the next. 
Lower Stuurtan System: As has been stated, true Bryozoa are first met with 
in the Chazy rocks of this system. In this group, excepting some of the calcareous 
strata in New York and Canada, originally referred here. the conditions were often 
quite unfavorable, not only for their preservation but for their development as well. 
In the excepted beds several species of Phylloporma and Rhinidictya belonging to 
the Cryptostomata, a considerable number of mostly undetermined Trepostomata, and 
Mitoclema, a genus of the Cyclostomata, have been found. Following the rocks west- 
ward from Canada the calcareous beds are lost, but the arenaceous portion, there 
known as the St. Peter sandstone, a formation totally unfitted for their preserva- 
tion, increases in thickness, and in Minnesota seems to be the only representative of 
the formation. The marble beds at Knoxville, Tennessee, which probably belong to 
the Chazy, are full of the remains of Trepostomata, none of which have, as far as we 
know, yet received critical study. 
Following the Chazy are the Birdseye and Black River limestones and shales. 
The first of these divisions has a wide geographical distribution, being known from 
New York and Canada to Tennessee and Kentucky, as a fine-grained, massive or in 
parts somewhat shaly limestone. The shaly layers are full of Bryozoa, among 
which the Cryptostomata are preeminently developed. In Minnesota the greater part 
of the “ Trenton limestone” and the lower two-thirds of the shales resting on it, are 
probably equivalent strata. Here the limestone is comparatively barren of Bryozoa, 
but the shales, on the contrary, are exceedingly rich, affording also a greater diversity 
